Inter-Amphibian Predation in the Early Cretaceous of China Lida Xing 1,2, Kecheng Niu3 & Susan E

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OPEN Inter-amphibian predation in the Early Cretaceous of China Lida Xing 1,2, Kecheng Niu3 & Susan E. Evans 4

For most fossil taxa, dietary inference relies primarily on indirect evidence from jaw morphology and the Received: 20 February 2019 dentition. In rare cases, however, preserved gut contents provide direct evidence of feeding strategy Accepted: 13 May 2019 and species interaction. This is important in the reconstruction of food webs and energy fow through Published: xx xx xxxx ancient ecosystems. The Early Cretaceous Chinese Jehol Biota has yielded several such examples, with lizards, birds, small dinosaurs, and mammals as both predator and prey. Here we describe an Early Cretaceous fossil frog specimen, genus Genibatrachus, that contains an adult salamander within its body cavity. The salamander is attributed to the hynobiid-like genus Nuominerpeton. The salamander skeleton is complete and articulated, suggesting it was caught and swallowed shortly before the frog itself died and was buried.

Assessing the diet of fossil organisms is important in understanding how they related to their environment and to other organisms within that environment, for example in reconstructing food-webs and energy fow within an ecosystem1,2. However, dietary inference in extinct organisms must usually be based on dentition and jaw archi- tecture, by comparison with living relatives with known feeding strategies. Tis can be challenging, especially for reptiles and amphibians with relatively simple dentitions. Gut contents provide the best direct evidence of diet in fossil taxa, and can yield surprises. Tus, for example, the simple conical teeth of the Early Cretaceous Chinese lizard Yabeinosaurus suggested insectivory1, but gut contents show it to have been a frequent piscivore3, indicating a rather diferent set of ecological interactions. Nonetheless, preserved and identifable gut contents are relatively rare, especially in small tetrapods. Invertebrate remains (insects, conchostracans) have been recorded in the guts of fossil salamanders from the Jurassic of China4,5, and indeterminate gut contents were noted in a salamander from the Early Cretaceous Spanish locality of Las Hoyas6. In fossil frogs, the record of gut contents is also most confned to fragmentary remains of insects, sponge or snail shell fragments, and plants7–9. Fossil evidence of amphibian predation on vertebrates is much rarer. Te only example we are aware of in a salamander is that of a 40–35 myr (Eocene) specimen of Phosphotriton sigei from France that contains frog remains10. In frogs, recorded vertebrate remains include fsh bones in an Oligocene palaeobatrachid from Germany11, indeterminate reptile bones in an Eocene frog from Messel, Germany12, and vertebrae of a larval frog in a specimen of the semi-aquatic Miocene Rana pueyoi from Spain8. Here we add to the record for frogs, with a specimen from the Early Cretaceous of China that has a complete adult salamander in its gut. Results Locality and horizon. The specimen, Yingliang Stone Nature History Museum (YLSNHM), YLSNHM01088, is represented by a part and counterpart block recovered from the Pigeon Hill locality, near Taipingqiao Village, Baoshan Town, Morin Dawa Daur Autonomous Banner of Hulunbuir City, Inner Mongolia, China (Fig. 1). Te deposits from which it was found are those of the Guanghua Formation that has been dated at 120–125 Ma13, and are thus stratigraphically and chronologically equivalent to the main fossil bearing beds of Yixian Formation in western Liaoning Province (Ar40/Ar39 dating14). Description and interpretation. Specimen YLSNHM01088 preserves the skeleton of a medium-sized frog (Snout-Pelvis length [SPL] = 73.6 mm, skull length 24 mm) in dorsal view (main block) with the sof tissue outline of the body clearly visible (Fig. 2a,b). Te frog skeleton matches the description given of Genibatrachus baoshan- ensis15, a pipanuran frog previously recorded from the Pigeon Hill locality, in the presence of unsculptured skull

1State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing, 100083, China. 2School of the Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, China. 3Yingliang Stone Nature History Museum, Nan’an, 362300, China. 4Department of Cell and Developmental Biology, University College London, Gower Street, London, WC1E 6BT, England. Correspondence and requests for materials should be addressed to S.E.E. (email: [email protected])

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Figure 1. Map of China showing the position of the Early Cretaceous Pigeon Hill locality in Inner Mongolia.

Figure 2. Specimen YLSNHM01088, a frog of the species Genibatrachus baoshanensis containing the skeleton of a salamander, cf. Nuominerpeton, in the body cavity. (a) Main part, skeleton in dorsal view, and (b) counterpart block. Te majority of the salamander skeleton is on the counterpart block (b).

bones, eight presacral vertebrae, procoelous vertebral centra, free ribs on presacral vertebrae 2–4, short, slender, fused ribs on presacrals 5–8, unexpanded sacral diapophyses, sickle-shaped clavicle with a lateral spike, coracoid with expanded proximal and distal ends, relatively short forelimbs (40% of hind limb length), a tibiofbula that

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Figure 3. Specimen YLSNHM01088. (a) Te skeleton of the salamander cf. Nuominerpeton, in ventral aspect, extracted from the counterpart block. (b) As (a), but with limb and jaw elements from the main block superimposed in red. Abbreviations: Ca.V, caudal vertebrae; Fe, femur; H, humerus; Ma, manus; Md.s, mandibular symphysis; Mx, maxilla; P, parietal; Pe, pelvis; Ps.V, presacral vertebrae; R/U, radius and ulna; Ta, tarsus; T/F, tibia and fbula.

is longer (115%) than the femur, and slender, unfused tibiale and fbulare of which the latter is slightly longer. Te skull is broad, and each maxilla carries around 50 small closely spaced teeth (40 in the holotype15). Te holotype specimen is also described15 as showing a stout robust body outline. Tis is consistent with specimen YLSNHM01088 where the well-preserved sof tissue outlines portray a heavily built frog with a broad body and thick, presumably strongly muscled, thighs and crura. Te holotype of G. baoshanensis was recorded as having an SPL of 70 mm and a skull length of 23 mm15. Specimen YLSNHM01088 is slightly larger. However, it appears to be less skeletally mature than the holotype, in that the ends of the long bones are unfnished and lack ossifed articular surfaces. In extant frogs, individuals of one gender (usually female) are ofen larger than the other so it is possible that whereas the holotype was skeletally mature, specimen YLSNHM01088 was of a diferent gender and still growing. Te salamander skeleton in the frog’s gut is most clearly visible on the counterpart block, where the skull, vertebral column, and some parts of the fore- and hindlimbs are preserved (Fig. 3a). More of the limb elements are preserved on the main block (Fig. 3b). Te salamander skeleton extends from under the frog’s anatomical lef shoulder girdle (skull), along the frog’s lef fank (forelimbs and anterior spine), and across the frog’s pelvic region ventral to the ilia and urostyle. Te salamander’s tail curls up along the right side of the frog’s abdomen but the distal end is missing. Te head is twisted in relation to the vertebral column so that the salamander skull, in ventral view, lies at roughly 90 degrees to the vertebral axis with the jaw symphysis close to the frog’s 5th and 6th presacral vertebrae (Fig. 2b). The only salamander currently known from the Pigeon Hill locality is Nuominerpeton aquilonaris16, a hynobiid-like species. We are unable to confrm any of the diagnostic characters listed for Nuominerpeton aqui- lonaris16, but the unsculptured skull bones, body proportions (axial length/limbs), vertebral shape, and visible limb morphology (e.g. humerus wider distally than proximally; well ossifed carpal and tarsal elements) of the ‘gut salamander’ are consistent with Nuominerpeton, and we provisionally refer it to that taxon. Te nine specimens of Nuominerpeton previously recovered from Pigeon Hill16 included four larvae (SPL 33.9–43.8 mm), one post-metamorphic juvenile (SPL 47 mm), and four adults (SPL 77.7–79.8 mm). The adults have exten- sive limb ossifcation compared to the juveniles, with a fully ossifed carpus and tarsus. Te ‘gut salamander’ is

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Figure 4. Reconstruction of the frog and salamander, roughly to scale, in silhouette. Te frog image is based on the extant Alytes, and the salamander image on the extant Hynobius.

somewhat telescoped and twisted, but it has an SPL of around 78 mm, which would correspond closely to adults of Nuominerpeton. Adult status is supported by the fully ossifed carpus and tarsus. Discussion Te Jurassic and Early Cretaceous deposits of north eastern China have yielded an exceptionally rich and diverse assemblages of plants, invertebrates, and vertebrates, many of which show exquisite preservation of hard and/or sof tissues. As a consequence of this fne preservation, these deposits have also yielded a signifcant number of specimens with gut contents. Tese include seeds in some birds (Jeholornis, Sapeornis17), insects and conchostracans in salamanders4,5, and several examples of vertebrate predation. As reviewed18, the predators (and their gut contents) include the mammal Repenomamus (juvenile psittacosaur); the birds Confuciusornis and Jianchangornis (fsh); the non-avian dinosaurs Sinosauropteryx (mammal), Sinocalliopteryx (Confuciusornis, Sinornithosaurus, indet. ornithischian dinosaur), and Microraptor (enantiornithine bird); the choristodere Hyphalosaurus (fsh); and the lizard Yabeinosaurus (fsh). Previous authors17,19–21 inferred that the Jehol amphibians fed predomi- nantly on insects and worms, and this would be a reasonable inference for Genibatrachus, given the many small, closely packed, teeth. However, frogs are opportunist feeders that take a range of foods, as demonstrated by YLSNHM01088. Extant terrestrial salamanders are eaten by a variety of predators including snakes, birds, small mammals, tur- tles, frogs, and other salamanders22,23, and they can represent a signifcant prey biomass in some environments23. Defence mechanisms include aposematic colouring, posturing, and unpleasant or toxic skin secretions24, but whether these were used by early salamanders is conjectural. Te salamander skeleton within YLSNHM01088 is largely intact with its bones in association. Tis suggests it had been caught and swallowed whole, apparently tail frst given the position of the skeleton (with the head lying proximally in the gut) and presumably still alive, not long before the frog died and was buried. Predator and prey were of comparable size (Fig. 4), and although the salamander was more gracile in its build, there must have been a struggle. Methods Te specimen was collected from the Pigeon Hill locality and is accessioned in the collections of the Yingliang Stone Nature History Museum (YLSNHM), Nan’an, China. Te specimen was digitally imaged at high resolution; the images of the part and counterpart blocks (Fig. 2) were then imported into Photoshop to digitally dissect the salamander skeleton from the background (Fig. 3a,b); and the bones from the two blocks were superimposed to form the composite (Fig. 3c). Te map in Fig. 1 was created with Surfer™, Version 725 and ArcGIS™, Version 9.226. References 1. Matsukawa, M., Shibata, K., Sato, K., Xu, X. & Lockley, M. G. Te Early Cretaceous terrestrial ecosystems of the Jehol Biota based on food-web and energy-fow models. Biol. J. Linn. Soc. 113, 836–853 (2014). 2. Buscalioni, A. D. et al. Te wetlands of Las Hoyas in Las Hoyas: a Cretaceous Wetland (eds Poyato-Ariza, F. J. & Buscalioni, A. D.) 238–253 (Verlag Dr Friedrich Pfeil, 2016). 3. Evans, S. E. & Wang, Y. New material of the Early Cretaceous lizard Yabeinosaurus from China. Cretac. Res. 34, 48–60 (2012). 4. Gao, K. Q. & Shubin, N. H. Earliest known crown-group salamanders. Nature 422, 424–428 (2003). 5. Dong, L. P., Huang, D. Y. & Wang, Y. Two Jurassic salamanders with stomach contents from Inner Mongolia, China. Chin. Sci. Bull. 57, 72–76 (2012). 6. Evans, S. E. Urodela in Las Hoyas: a Cretaceous Wetland (eds Poyato-Ariza, F. J. & Buscalioni, A. D.) 138–142 (Verlag Dr Friedrich Pfeil, 2016). 7. Leal, M., Brito, P. & Martill, D. Anurans of the Crato Formation in Te Crato Fossil Beds of Brazil: Window into an Ancient World (eds Martill, D., Bechly, G. & Loveridge, B.) 444–451 (Cambridge University Press, 2007). 8. McNamara, M. E. et al. Soft-tissue preservation in Miocene frogs from Libros, Spain: insights into the genesis of decay microenvironments. Palaios 24, 104–117 (2009). 9. McNamara, M. E. et al. Exceptionally preserved tadpoles from the Miocene of Libros, Spain: ecomorphological reconstruction and the impact of ontogeny upon taphonomy. Lethaia 43, 290–306 (2010).

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